In one type of acoustic device, such as an open fitting hearing aid, a vent connecting the inside of the external ear canal to the outside is provided to alleviate an occlusion effect when wearing the hearing aid (for example, see JP 2006-304147 A (PTL 1)). This type of hearing aid includes a microphone, an earphone, and a vent. The microphone collects sound from a sound source, and the earphone causes the user to hear the sound collected by the microphone. The vent is a hole connecting the inside of the external ear canal to the outside, as described above. As a result of the vent, the external ear canal is not completely sealed. Therefore, the occlusion effect that occurs when wearing the hearing aid is alleviated.